Large motion color images, such as displayed in movie theaters, are formed by projecting light through individual film frames illuminating a full screen, with frames succeeding one another at 20 to 30 times a second. Movie projection utilizing an electronic (usually digital) image source (termed “video” herein) is a desirable alternative to film, assuming such an image can be projected with sufficient brightness, resolution, color balance, registration, and lack of motion artifacts to equal or exceed the capabilities of film.
The typical prior art laser projection systems used complicated lens and mirror systems to combine modulated colored beams into a composite beam to be scanned, and additional optics to scan and focus the beams onto a screen. These optics sap much of the power of the laser beams, making laser projection images substantially less bright than conventional film images.
Laser video-projectors have been used for the display of electronic images since about 1980, with the first projector built in England by the Dwight Cavendish Company. This projector used an Argon ion laser and a dye laser to produce standard television resolution images up to about ten feet across in a darkened room. The projector was very large and was difficult to operate. The Dwight Cavendish laser projector, and indeed any laser projector, required the following basic components to make a video image: (a) lasers to supply the light that is sent to the screen to form the image; (b) a method of controlling the intensity of the laser light for each portion of the image, often called “modulation”; and (c) a method of distributing the modulated light across the screen surface, often called “scanning”.
An improved version of the Dwight Cavendish laser projector is described by Richard W. Pease in “An Overview of Technology for Large Wall Screen Projection using Lasers as a Light Source”, MITRE Technical Report, The MITRE Corporation (July 1990). The projector described in the MITRE publication utilized the following components corresponding to the laser source, modulator and scanner described above. The laser sources included argon ion lasers to produce 454 to 476 nm blue and 514 nm green, and Rhodamine 6G dye laser pumped with an argon ion laser to produce 610 nm red. The system used acousto-optic modulators between the laser sources and the scanning component for the laser beam of each color, with the modulated beams later combined with dichroic mirrors and deflected and focused onto the scanning component. The scanning section included a rotating polygon mirror and galvanometer-controlled frame mirror, as further described below. The rotating polygon mirror had 25 mirror facets, each of which deflected the modulated beam horizontally across a predetermined angle onto a mirror tilted vertically by a galvanometer across a predetermined angle through lenses onto the screen.
Several problems in particular limit the ability of current large screen projection technology to produce movie theater quality laser images. Because such laser projection systems typically used complicated lens and mirror systems to combine modulated colored beams into a composite beam to be scanned, and to scan and focus beams onto a screen, much of the power of the laser beams was sapped away, making laser projection images substantially less bright than that produced by film projection. Further, because certain wavelengths, especially blue, have been difficult to produce at adequate power levels with lasers, brightness and color balance have been inadequate for large screen video applications. The complex optics and scanning systems also tended to cause color separation and image artifacts. Also, projection systems that used rotating polygon mirrors did not adequately address the problems of polygon facet pointing errors that would tend to slightly misdirect the beams, thus requiring additional complex optical or mirror array systems to compensate for the slight misdirections.
Perhaps the most significant problem, however, with prior laser projections systems in comparison with film projection technology is the lack of sufficient resolution. Attempts to increase resolution only exacerbated the problems noted above. In order to effectively compete with or displace film projection, it is widely believed that laser projection systems must be capable of resolutions approaching 1900 by 1100 fully resolved pixels, or roughly the maximum resolution of High Definition Television (HDTV) standard of 1920×1080p.
Standard television quality resolution rarely exceeds 525 horizontal lines repeated 30 times a second. For television to achieve this resolution, 525 horizontal lines of analog image data are scanned, roughly comparable to a digital pixel array of 525×525 pixels. Thus, television quality video would require the scanning of more than 945,000 lines per minute. A 25 facet polygon mirror writing one line with each facet would require a rotation of more than 37,500 rpm. Because of centrifugal force limitations, rotational speeds this high limit the feasible size and/or number of the facets.
If one were to attempt scanning 1920×1080 HDTV or better resolution video with prior art projectors the increased number of lines per frame would require either an increase in the number of facets or substantially increased polygon mirror rotational speeds. Further, such a system may also require larger facets further straining centrifugal force limitations. For HDTV 1920×1080p resolution at a full frame rate of 60 frames per second, this polygon would have to scan more than 3.8 million lines per minute, and achieve a rotational speed of more than 150,000 rpm. A polygon mirror assembly capable of these facet rates would be structurally difficult to manufacture and operate, and extremely expensive.
The limitations of modulation technology pose additional problems. Each laser beam of the three primary colors must be modulated to produce a different color intensity for each pixel being scanned. For standard television resolution, more than 250,000 modulations must occur for each frame for each color or laser, or a total of 7.5 million modulations per second for 30 full frames per second. For high resolution, at 1920×1080p, more than 2 million modulations must occur for each color or laser to scan each frame, or a total of at least 120 million modulations per second per color for 60 frames per second. For desired non-interlaced (progressive) imagery having even greater resolution, such as 3000×2000 pixels, the rate is above 360 million modulations per second. Current modulation technology as used in prior art laser projectors is not capable of modulating the laser beams, especially powerful laser beams, at a sufficient rate to enable the generation of the number of discreet pixels required for even film-quality digital resolution.
There are other inadequacies in the existing technology that are not addressed in detail here that impose additional challenges, including complexity of optics, brightness, resolution, contrast and image stability.